The invention relates generally to electronic totalizers, and more particularly to a totalizer that incorporates adjustable scaling means making it possible to establish output increments in units appropriate to the use to which the totalizer is put, such as units in terms of gallons, liters or tons.
While a totalizer in accordance with the invention is usable in conjunction with any form of transducer responsive to a process variable to produce an output signal in a standard industrial process control range, such as 4 to 20 mAdc, the totalizer will be described in the context of a two-wire transmitter that includes a field-mounted transducer for converting a process variable, such as liquid level or flow rate, into a signal lying within a standard current range. This signal is conveyed over the two-wire line to a remote receiving station in which the signal is indicated or recorded, or used to operate process controllers. An important advantage of a two-wire transmitter is that the line which conveys the current signal from the field transducer to the remote station also acts to deliver d-c power from a power supply at the station to the transducer, thereby obviating the need for extra power lines.
In many industrial applications for a two-wire transmitter, the output value in rate form must be totalized for billing or other purposes to determine the total flow that has passed through the meter. In the Shauger et al. U.S. Pat. No. 4,158,765, whose entire disclosure is incorporated herein by reference, an electronic totalizer is disclosed for use in conjunction with a two-wire transmitter whereby the line carrying current from the transducer to the remote receiving station also supplies power to the totalizer as well as to the transducer. The totalizer provides a non-destructive readout of the metered variable, yet its operation has no adverse effect on the normal behavior of the transmitter.
The input circuit of the Shauger et al. totalizer includes a resistor through which the line current signal passes to produce a proportional voltage which is applied to a resettable integrator yielding a time-integrated voltage that rises above a base level at a rate depending on the varying magnitude of the voltage applied thereto. This time-integrated voltage is fed by a comparator set by a reference voltage to produce an output signal only when the integrated voltage reaches a threshold. The output signal generated by the comparator is fed back to the integrator to reset it to base level and it is also applied on a trigger to a one-shot to produce a pulse. The pulse is applied to a digital counter whose accumulated count represents the time-integrated total of the metered variable.
Thus if the meter measures the flow rate of a process liquid, the Shauger et al. totalizer will produce, say, one output pulse for every 100 gallons of flow. When, therefore, the count reads 30, this simply means that a total of 3000 gallons has passed through the flowmeter.
Though the Shauger et al. totalizer has many advantages over prior art totalizers, it lacks the ability to scale its output digitally to any desired rate. Thus if the totalizer is arranged to totalize flow in terms of gallon units, one is not able, at will, to readjust the totalizer to provide a readout in liters, in tons or in other measuring units.